This invention relates to the disintegration of wood, and more particularly to methods of and apparatus for reducing a body of wood to a multiplicity of particles.
The invention is especially concerned with the disintegration of bodies of wood, and more particularly debarked logs, to produce wood particles for production of wood pulp for making paper.
Heretofore, generally standard practice for making wood pulp for paper production has involved the mechanical comminution of debarked logs, utilizing knives for chipping the logs, or grinding wheels or grindstones for grinding the logs. In either case, relatively high initial equipment cost and relatively high energy (power) consumption are involved, and the knives need sharpening and the grindstones need dressing, which is time-consuming and costly. Not only that, cutting and grinding may produce crushed fibers, which may be detrimental to the production of good quality pulp.
It is understood that much of the newsprint currently used is made from a mixture of 70% to 80% ground wood and the remainder unbleached sulphite or semi-bleached sulphate pulp. While ground wood pulp is of lower cost than chemical pulp, the cost of ground wood pulp is still relatively high because the grinding operation involves relatively high power consumption. Use of ground wood pulp may also have the disadvantage that paper containing a high percentage of ground wood is adversely affected as to color and strength qualities by exposure to sunlight, heat and air, and, is therefore less desirable for use in making newsprint, which must be capable of being fed through modern high speed presses without web breakage, and also capable of accepting inks with good printability.
With regard to energy consumption involved in grinding wood, energy is usually expressed in terms of horsepower/tons per day of air-dried pulp produced, i.e. horsepower input divided by the tons of pulp produced per day. The energy supplied to the grindstone is consumed in overcoming the friction between the stone and the surface of the wood being ground, and it is believed that almost all of it is transmitted to sensible heat in the water which is sprayed on the stone, and that only a small amount of the energy is absorbed in forming new surfaces as particles (fibers) separate from the wood. For example, in "Wood Machining Processes", by Peter Koch, published by Ronald Press Company, New York, N.Y. 1964, the average power requirement to yield ninety to ninety-five percent fiber from the original log volume is stated as 65 to 75 horsepower per day per ton (on an oven dry basis). According to "Pulp and Paper", by James P. Casey, published by InterScience Publisher, Inc., New York, N.Y. 1952, at a grinding pressure of 20 psi, the power consumption for a number of species of wood is as follows:
Spruce: 70 hp/day/ton, PA1 Hemlock: 108 hp/day/ton, PA1 Jack Pine: 105 hp/day/ton, PA1 Shortleaf Pine: 125 hp/day/ton, PA1 Poplar: 140 hp/day/ton, PA1 Cottonwood: 215 hp/day/ton.